1. Field of the Disclosure
The invention relates to a field effect transistor made of a nitride compound semiconductor.
2. Description of the Related Art
Semiconductor devices, for example, GaN semiconductor devices using a group III-V nitride compound semiconductor represented by AlxInyGa1-x-yAsuPvN1-u-v (0≦x≦1, 0≦y≦1, x+y≦1, 0≦u≦1, 0≦v≦1, u+v<1) have a large bandgap energy and a good heat resistance and thus are excellent in high temperature operations compared to GaAs semiconductor devices. Especially, a field effect transistor (FET) using GaN/AlGaN semiconductors has been developed. In a horizontal device, a MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) structure is used to realize normally-off type operations. Because a field effect transistor having this structure is operable at a high current, its application to a power device is expected.
For example, International Patent Publication No. 2003/071607 discloses a normally-off field effect transistor having the structure called hybrid MOSHEMT (Metal-Oxide-Semiconductor High Electron Mobility Transistor). This field effect transistor has a HEMT structure in which a carrier traveling layer formed of an undoped GaN semiconductor and a carrier supplying layer formed of an AlGaN semiconductor are sequentially stacked on a p-type layer formed of a p-type GaN semiconductor. Two-dimensional electron as having a high mobility is used as carrier to realize a low on-resistance, in which the electron gas is generated near the heterojunction interface of the carrier traveling layer with the carrier supplying layer. The field effect transistor forms a recess by etching the carrier supplying layer and the carrier traveling layer only in the vicinity of a gate in order to realize normally-off type operations.
In the field effect transistor, because the p-type layer and the carrier traveling layer have a RESURF structure and thus relieve a local concentration of an electric field, a high withstand voltage can be achieved. The withstand voltage characteristics of the RESURF structure depends on a balance between the p-type carriers in the p-type layer and the two-dimensional electron gas carriers in the carrier traveling layer.
In order to realize a high withstand voltage, a technique is disclosed for providing a semiconductor layer formed of an AlGaN semiconductor in addition to the carrier supplying layer to form a double hetero structure (see “D. Visalli, et al., Solid State Device Meetings 2008-proceedings, No. G-1-2”).
However, when a carrier concentration of the two-dimensional electron gas is enhanced to improve the on-characteristic in the normally-off field effect transistor having the hybrid MOSHEMT structure, it becomes difficult to keep a balance of the carriers in the RESURF structure and thus difficult to maintain the withstand voltage. Accordingly, it is difficult to keep low on-resistance characteristics and high withstand voltage characteristics in a well-balanced way.